Self-assembly of amphiphilic fluorescent dyes showing aggregate-induced enhanced emission: Temperature dependence of molecular alignment and intermolecular interaction in aqueous environment

Article abstract of DOI:10.1039/b910531j

A 1-cyano-1,2-bis(biphenyl)ethene derivative having hexa(ethylene glycol) groups as amphiphilic side chains showed aggregate-induced enhanced emission in water; upon heating the intensity of the enhanced emission was attenuated in a lower temperature range than the temperature range where exciton interaction is still effective.

Full text of DOI:10.1039/b910531j

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Self-assembly of amphiphilic fluorescent dyes showing aggregate-induced
enhanced emission: temperature dependence of molecular alignment and
intermolecular interaction in aqueous environmentw
Takashi Hiroseab and Kenji Matsuda*ab
Received (in Cambridge, UK) 28th May 2009, Accepted 20th August 2009
First published as an Advance Article on the web 8th September 2009
DOI: 10.1039/b910531j
A 1-cyano-1,2-bis(biphenyl)ethene derivative having hexa(ethylene
glycol) groups as amphiphilic side chains showed aggregate-
induced enhanced emission in water; upon heating the intensity
of the enhanced emission was attenuated in a lower temperature
range than the temperature range where exciton interaction is
still effective.
investigated their self-assembling behavior and fluorescence
enhancement properties in water. Compound 1 is simply an
example of general fluorescent dyes that show quenching upon
aggregation. Temperature dependent UV-Vis, induced CD,
and fluorescence spectra, indicate a stepwise change of
supramolecular environment of 2.
PEAnt dyes possess quite high fluorescence quantum yield
in dilute solution7 while CNBE dyes exhibit AIEE properties.5
Both dyes have similar molecular size and structural stiffness.
Poly(ethylene glycol) (PEG) chains having appropriate length
(n = 6) for surrounding the hydrophobic core moiety were
introduced into both terminals of the dyes as hydrophilic side
chains.8 In order to induce a chiral environment in the
self-assembled structures and to probe the chiroptical properties,
a methyl group was introduced nearby the aromatic core
moiety as a chiral source.
Recently, high-efficiency fluorescent materials in the
condensed phase have attracted considerable interest among
scientists because of their potential application for organic
light-emitting diodes (OLEDs), optical sensors, biological
probes,1 and so on. In the case of most organic fluorescent
dyes, however, the emission is strongly quenched when the
dyes form aggregates in the condensed phase, mainly because
intermolecular vibronic interactions induce nonradiative
deactivation process.2
Both amphiphilic compounds 1 and 2 were soluble in a wide
range of solvents such as hexane, ethyl acetate, methanol, and
even in water. The UV-Vis absorption spectrum was clearly
different in water as compared with that in other organic
solvents (Fig. 2(a) and (b)). The absorption maximum of 1
was red-shifted in water whereas that of 2 was oppositely
blue-shifted by 10 nm from 365 nm in ethyl acetate to 355 nm
in water. In both cases, the absorption bands are broadened
and absorption edges are extended to longer wavelength in
water than in the organic solvents. The distinctive spectral
change in water suggests the operation of intermolecular
electronic interaction or the formation of aggregates (such as
H- or J-type aggregates9) in water.
In order to overcome the problem of quenching in the
condensed phase, several strategies have been proposed. The
introduction of a bulky substituent can be a strategy for
retaining highly fluorescent properties in the condensed
phase by preventing unfavorable aggregation and decreasing
intermolecular interaction.3 The second strategy is utilizing a
unique luminescent property originating from an aggregated
state. Some dyes show intense enhanced fluorescence in the
aggregated state whereas they exhibit weak or almost no
emission in dilute solution. This unique phenomenon is widely
researched as aggregate induced enhanced emission (AIEE) or
aggregate induced emission (AIE).4
Recently, Park and co-workers have reported interesting
AIEE properties of cyano-bis(biphenyl)ethene (CNBE)
derivatives, which involves not only fluorescence enhancement
but also the significant red shift of emission along with the
formation of nanoparticles.5 The specific emission from the
aggregated state can be a suitable probe to evaluate the change
of microscopic environment.
The formation of self-assembled structures in water was
strongly supported by CD spectrum measurements (Fig. 2(c)
and (d)). Both 1 and 2 exhibited exciton-coupled CD signals in
water whereas they were CD inactive in ethyl acetate. The
result indicates that compounds 1 and 2 self-assemble into
structures in water where exciton interaction occurs between
transition dipole moments of adjacent molecules located
on spatially twisted positions.10 In ethyl acetate, they are
considered to be molecularly dispersed due to good solubility
of the core moiety. The intensities of exciton-coupled CD
signals are very different between compounds 1 and 2. The CD
In this study, we synthesized 9,10-bis(phenylethynyl)anthracene
(PEAnt) derivative 1 and CNBE derivative 2 having hexaethylene
glycol (Hxg) groups as amphiphilic side chains6 (Fig. 1) and
a Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, JST, CREST,
Katsura, Nishikyo-ku, Kyoto, 615-8510, Japan.
E-mail: kmatsuda@sbchem.kyoto-u.ac.jp; Fax: (+81) 75-383-2739
b Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka,
819-0395, Japan
w Electronic supplementary information (ESI) available: Full
experimental procedure, 1H NMR spectra, FL lifetime measurement
of 2, additional TEM images. See DOI: 10.1039/b910531j
Fig. 1 Molecular structures of FL dye derivatives having Hxg side
chains.
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Fig. 2 UV-Vis absorption and CD spectra of 1 (a, c) and 2 (b, d) in
ethyl acetate (gray dashed line) and in water (black solid line);
c = 3 Â 10À5 M (in water) and 1 Â 10À5 M (in ethyl acetate) for 1,
and c = 2 Â 10À5 M (in water) and 8 Â 10À6 M (in ethyl acetate) for 2.
Fig. 3 (a) TEM image of 2. (b) Small-area image of 2. (c) DLS size
distribution of 2 in water. (d) A calculated molecular model of
compound 2 by semi-empirical MO method (AM1/MOPAC).
signal of an aqueous solution of 2 was very strong relative to 1
and other similar derivatives reported previously.11 The CD
spectrum of 2 has a typical feature of an exciton-coupled CD
signal separated by a Davydov splitting; the first positive
self-assembled structures (Fig. 4(a)). For CNBE derivative 2,
the fluorescence quantum yield was Ff = 0.0062 in ethyl
acetate and Ff = 0.021 in water. Thus, about 3.4-fold
enhancement of fluorescence efficiency was observed for 2
along with self-assembly in water (Fig. 4(b)). Furthermore,
the maximum wavelength of fluorescence was red-shifted from
456 to 497 nm, and a bright green color was seen in water
compared to dark blue in ethyl acetate. The behaviors of
UV-Vis, induced CD, and fluorescence spectra were essentially
the same in the range of concentrations measured (see ESIw).
Fluorescence lifetime experiments show that the lifetime of 2
in water was significantly longer than that in ethyl acetate (see
ESIw). Considering that J-aggregates in which excitons
are delocalized over a number of molecules have shorter
fluorescence lifetime, the emission of 2 in water is not
considered to originate from such ideal J-aggregates.9,13
Cotton effect was observed at 378 nm (De
= 109.5,
g = 0.50%) and the second negative one at 339 nm
(De = À76.7, g = 0.37%).12 Of note, the strength was
comparable to the situation of the intramolecular exciton
coupling of two dimethylamino-benzoate groups fixed in a
cholesterol framework.10 The strong induced CD suggests the
existence of suitable alignment of transition dipole moments
and the strong exciton interaction in the supramolecular
structure of compound 2 in water.
To obtain more information on the aggregated structure
that shows a distinctively strong CD intensity, the supra-
molecular structure of 2 was evaluated by means of transmission
electron microscopy (TEM), dynamic light scattering (DLS),
and molecular modeling (Fig. 3). A TEM image of compound
2 showed hierarchical fiber-like structures. The size of one of
the narrowest fibers was around 15 nm and other typical
thicker fibers have diameters of 35 or 75 nm. The narrowest
one was comparable with the molecular size of 2 that was
calculated to be 7.1 nm by a semi-empirical MO method
Poly(ethylene glycol) is
a thermoresponsive polymers
showing LCST (lower critical solution temperature) behavior.14
LCST is a type of phase transition phenomenon, which is
usually explained on the basis of the thermal cleavage of
hydrogen bonds between hydrophilic moieties and water
molecules due to an entropic effect.
Amphiphilic compounds 1 and 2 possessing Hxg side chains
exhibited a LCST transition in water. An aqueous solution of
2 turned turbid upon heating and the absorption spectra
dramatically changed along with the transition (Fig. 5(a)).
By monitoring the change of absorbance at 600 nm, where the
(AM1/MOPAC). The observation suggests that
a few
molecules aggregate with each other and construct a piece of
the fundamental narrowest fibers. The size distribution of
aggregates in an aqueous solution of 2 measured by DLS
exhibited three peaks, whose maxima were observed at 5.9,
82.4 and 2.9 mm. The largest peak, assignable to micro-
meter-size structures, could be removed by filtration, however,
a similar peak regenerated after standing for several minutes.
These results support the existence of hierarchical structures
and a large size distribution of self-assembled structures in
aqueous solution.
In order to investigate fluorescence properties along with
self-assembly, fluorescence spectra of 1 and 2 were evaluated in
both solvents (Fig. 4). PEAnt derivative 1 showed a brilliant
green emission in ethyl acetate (Ff = 0.77), whereas the
emission was strongly quenched in water (Ff = 0.0061)
probably due to self-quenching along with the formation of
Fig. 4 Fluorescence spectra of (a) 1 (lex = 450 nm) and (b) 2
(lex 354 nm) in ethyl acetate (gray dashed line) and in water (black solid
line). The absorbance at the excitation wavelength was set to 0.09 for all
samples. Photographs of solutions of 1 and 2 in ethyl acetate (left) and in
water (right) upon irradiation with 365 nm light are shown as insets.
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the microscopic change of molecular location and to the
successive large structural change in the LCST transition.
Furthermore, the enhanced emission originating from the
aggregated state of 2 decreased faster than the CD signal.
The result indicates that the AIEE behavior is more sensitive
to the molecular environmental change than the exciton
interaction, suggesting that the AIEE behavior can be used
as a sensitive probe for the microscopic environmental change.
This work was supported by CREST, JST and by a
Grant-in-Aid for Young Scientists (A) (No. 19685013) and a
Grant-in-Aid for Science Research in a Priority Area ‘‘New
Frontiers in Photochromism’’ (471) (No. 19050009) from the
MEXT, Japan. T. H. acknowledges JSPS for the young
scientist fellowship.
Notes and references
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